The present invention concerns a device for monitoring a living object during a magnetic resonance (MRI) experiment in an MRI tomograph, wherein the device comprises one or more individual electrodes which are connected to the living object to be examined in an electrically conducting fashion, and are connected to a monitoring device via signal lines, wherein each signal line comprises individual parts that are electrically connected to each other via impedances.
A device of this type is disclosed in U.S. Pat. No. 4,951,672 A (Reference [7]).
MRI systems are widely used medical and diagnostic devices. The primary components of an MRI system are the magnet that generates a stable and very strong magnetic field (B0), the gradient coils that generate an additional variable magnetic field, and the RF antennas that are used to send energy into the measuring object and to receive the NMR signal from the measuring object. A computer controls the overall process and is required to process the received information.
The object, on which the MRI measurement is carried out, is called patient below. This term explicitly also includes an animal.
In some cases, the patient must be monitored during an MRI examination. This may be required for medical reasons (e.g. for monitoring breathing, the blood oxygen, the body temperature) or also for synchronizing the individual scans of the MRI experiment with physiological changes in the patient (e.g. ECG, EEG or breathing).
MRI experiments, during which the patient is monitored in some way, are called “monitored MRI experiments” below.
For monitored MRI experiments, monitoring electrodes are typically attached to the body of the patient. The electrodes are connected to the monitoring device via signal lines, wherein the monitoring device, in which the signal received by the electrodes is processed and displayed, is often located outside of the electromagnetically shielded MRI region.
The patient is monitored or the MRI experiment is triggered during the MRI measurement. This means that RF pulses or gradient pulses can induce currents in the signal lines. This happens, in particular, when the lines are directly guided through the RF antenna or the gradient, which is often unavoidable.
Coupling Between Gradient and Signal Lines
Currents are induced in conductor loops (e.g. first electrode—body of the patient—second electrode—equivalent impedance of the signal lines) which are permeated by the changing gradient field, wherein the currents can cause heating (e.g. of the contact point between the electrode and the body) and thereby burn the patient.
Coupling Between the RF Antenna and the Signal Lines
The signal lines themselves directly act as antennas which receive the electromagnetic field of the RF antenna. This is called coupling between the signal lines and the RF antenna. Depending on the length and position of the signal lines, this coupling may have a varying strength. During transmission, i.e. during transmission using the RF antenna, i.e. when power is radiated into the system, strong coupling between the RF antenna and the signal lines causes induction of large currents in the signal lines, which, in turn, heats the signal lines, which, in the worst case, can result in burning the patient.
One fact that was neglected in previous publications is that coupling between the RF antenna and the signal lines causes B1 field distortions in the vicinity of the signal lines, causing artefacts in the MRI image or, in the extreme case, also burning the patient. This happens e.g. when the B1 field distortion generates increased local fields in the body of the patient, with the consequence that excessive RF power is deposited at this location, thereby generating an excessive amount of heat. During reception, i.e. when the RF antenna receives signals from the excited nuclear spins, coupling between the RF antenna and the signal lines, in turn, results in inhomogeneous illumination. In the special case, when a phased-array coil is used as an RF antenna, coupling of the individual array coils with the signal lines increases the crosstalk between the individual RF channels and therefore deteriorates the signal-to-noise ratio of the MRI measurement.
Van Genderingen et al (Radiology 1989) describe the use of carbon fibers as ECG lines. The predominant object thereby is to reduce field disturbances of the gradient field, and the associated artefacts when metallic ECG cables are used. The use of carbon fiber ECG lines has become common practice in the meantime. The resistance of carbon fiber lines is in a range of a few hundreds of ohms per meter. This resistance reduces image artefacts due to field interferences of the gradient field, but is not sufficient to prevent heating of the ECG lines due to RF currents or prevent coupling between the RF antenna and the ECG lines to a satisfactory degree.
There are a plurality of publications that follow this idea and try to increase the ohmic resistance of the signal lines by some means.
References [1] to [3] e.g. describe a signal line that is produced through thin film technology and has a resistance of 10,000 ohm/ft. This, however, only deals with the distributed DC resistance of the line.
Reference [4] also describes an ECG line which is especially designed for MRI applications and is intended to prevent heating of the ECG line or the electrode. The ECG line in this case is a spirally wound nickel chromium (Nichrome) wire which has a resistance of 7.5 to 30 kOhm/meter. This patent describes the problem that, when high ohmic resistances are used in the ECG lines, these must be exactly matched to each other to prevent problems with the common mode noise rejection of the ECG device. The described ECG cables are therefore very complex and expensive.
References [5]-[7] describe introduction of one or more discrete ohmic resistances into the signal lines. Reference [7] describes an ECG line that is especially designed for MRI applications, as shown in FIG. 1, which should prevent heating of the ECG line or the electrode. Reference [7] claims that, towards this end, the ECG line must have a high DC resistance and describes how resistances of a magnitude of 33 to 100 kOhm are integrated in the electrode or the ECG cable. The document describes the possibility of dividing the desired resistance into smaller resistances and distributing them over the length of the ECG cable. The central importance of the position and the separations at which the resistances must be disposed is, however, not recognized. Moreover, this patent does not comprehend the coupling mechanism between the ECG line and the RF antenna.